The present invention relates to a lubricating system for a horizontal compressor, and more specifically to a pressure differential lubricating system for a rotary rolling piston and horizontal crankshaft hermetic compressor, which are usually used in small refrigerating systems.
Horizontal compressors of the rotary rolling piston type have been more and more used in refrigerating appliances because they make possible, comparing with the vertical model, an additional gain in terms of useful volume for the refrigerator.
The oil circulation in horizontal crankshaft compressors cannot occur, according to the technique normally used in vertical crankshaft compressors. This technique includes providing a centrifugal pump at the shaft lower extremity which is immersed in the sump reservoir placed at the bottom of the shell with the pump pushing the oil through the shaft up to the parts which require lubrication. For lubrication of horizontal crankshaft compressors, it is necessary to provide means to cause the oil to rise from its sump up to the shaft, where it is distributed through the bearings and other parts to be lubricated.
One of the known ways for oil elevation and circulation is disclosed in U.S. Pat. No. 4,449,895. That patent describes a rotary horizontal crankshaft hermetic compressor whose lubricating system comprises a curved tube which is extended up to the oil sump placed at the bottom of the shell, and a helical spring which rotates in its interior. The spring has one of its ends connected to the eccentric rotating piston shaft while its other end remains immersed in oil.
When the eccentric shaft is driven, it transmits rotation to the spring, causing the oil to be elevated through the annular passage created between the spring coils and the tube internal surface. The oil is, in such case, is conducted up to the pressure chamber placed at the sub bearing extremity, being then distributed along it as well as the eccentric shaft and main bearing by means of grooves made on the shaft surface.
In spite of assuring a continuous supply of oil for the bearings and eccentric shaft, the spring arrangement has the inconvenience of causing additional mechanical losses in the compressor due to the energy dissipation which occurs by friction between the spring coils and the tube internal surface.
Another inconvenience of this approach is that the shell must necessarily have a longer length due to the necessity of larger internal space for the assembly of the oil tube at the sub bearing extremity. This length increase, besides requiring a greater quantity of material (steel plate) for the shell conformation, results in a greater suction gas superheating with consequent drop of compressor volumetric efficiency. The superheating occurs due to the heat flux of compressed gas discharged in the interior of the shell at high temperature for the suction gas, which is admitted to the system through a connecting tube internal to the shell. The longer the tube length, the greater is the heat flux through it and, consequently, the suction gas superheating.
Another disadvantage of the previous technique concerns the cost of the spring manufacture, which tends to be raised due to the non-circular transversal section of the wire that requires a specifically made spring from the manufacturer.
Another known way for elevating and circulating oil is disclosed in U.S. Pat. No. 4,472,121. This patent describes a rotary horizontal type compressor provided with a lubricating system in which the lubricating oil accumulated at the bottom of the shell is supplied through a lubricating hole made in a central and axial way on the eccentric shaft by the effective use of refrigerant gas pulsation at high pressure discharged from the compression chamber. Therefore, the compressor is provided with: a lubricating oil feeding tube having one end in communication with the lubricating hole of the eccentric shaft and the other end open to the lubricating oil in the sump. There is also a refrigerant gas discharge tube, having one end inserted at the end of the lubricating oil feeding tube open to the sump, and the other end in communication with the refrigerant gas discharged from the compressor chamber.
When the refrigerant gas is discharged from the discharge tube inward the end of the oil feeding tube, the one open to the sump, the lubricating oil accumulated at the bottom of the shell and mixed with the refrigerant gas is carried inward along the lubricating oil feeding tube through a passage formed at the connection of the ends of the two tubes being then supplied in an oil collector and distributed through the central lubricating hole in the parts to be lubricated. Despite its simple construction and low cost, this system presents the inconvenience of causing oil foaming due to the refrigerant absorption, reducing the viscosity of the lubricant and altering, consequently, the bearings lubricating conditions.
U.S. Pat. No. 4,568,253 discloses an oil pump for a rotary horizontal crankshaft hermetic compressor whose bearing is provided with a channel vertically arranged in communication with the oil sump. The eccentric shaft has a portion of reduced diameter which forms with the bearing an annular chamber. Several helical grooves are provided arranged in an angularly opposite way and in communication with the annular chamber.
The eccentric shaft rotation develops a low pressure zone at the annular chamber causing the lubricant to be suctioned upward through the bearing channel and the annular chamber alignment. The helical grooves distribute the annular chamber lubricant to the end portions opposite to the eccentric shaft, lubricating the bearings and other movable parts of the compressor.
In spite of presenting a simple and low cost construction, this type of pump also presents in practice some inconvenience. The helical grooves made on the shaft end portions reduce the sustaining effective area of the bearing, already reduced by the recessed intermediate portion of the shaft, causing contact and, consequently, the wear of the eccentric shaft and the bearing.
Another problem is that the oil flux in this system is seriously affected by the presence of gas refrigerant in the system, which occurs primarily at the compressor start-up time. This gas refrigerant is released from the oil during the compressor stand-still periods, creating bubbles that are kept at the bearing and oil feeding channel. At the compressor start-up time, the depression created between the shaft and the bearing causes expansion of those bubbles, giving rise to a certain delay in the suction and oil supply at the bearing, thereby reducing its lubrication effect.
Another known way of elevating oil up to the crankshaft is described in U.S. Pat. No. 4,624,630 which is accomplished by using the difference of pressure between the cylinder internal volumes, especially the suction one, and the high pressure inside the shell.
The oil in the sump is forced by the fluid refrigerant which is discharged at high pressure in the interior of the shell, draining the oil through a hole that connects the oil sump to the interior of the cylinder. The hole is usually in the space between the crankshaft eccentric side and the bearing cover wall, and communicates with various lubricating channels. This solution, although extremely simple, has the inconvenience of not giving full lubrication to the channels, even in normal operating conditions. In conditions of small pressure differential the lubrication can be substantially reduced.